Method and apparatus for controlling an electromagnetic clutch for use on a vehicle

ABSTRACT

A method and an apparatus of controlling an electromagnetic clutch incorporated in a power transmission system of a vehicle to transmit an output of an engine to the drive wheels of the vehicle is disclosed. An amount of electric current applied to the clutch is regulated to control an engaging action of said electromagnetic clutch, based on either a variation in rotating speed or torque of an output shaft of the electromagnetic clutch, or based on the frequency or number of variations per unit time in rotating speed or torque of the output shaft, as compared with the frequency or number of ignitions or combustions per unit time of the engine. In this way a variation in output torque of the engine is absorbed by the electromagnetic clutch for improved drivability, while a transmission power loss in the clutch is kept to a minimum for maximum fuel economy.

BACKGROUND OF THE INVENTION

The present invention relates in general to a method and an apparatusfor controlling an electromagnetic clutch for use on an automotivevehicle, and more particularly to an improved technique by which theelectromagnetic clutch is controlled so as to absorb a variation inoutput torque of the engine for improved drivability of the vehicle, yetwith a minimum power loss in the electromagnetic clutch for economicaloperation of the engine.

For improved fuel economy of a vehicle engine, it has been known in theart to operate the engine at a relatively low speed with a relativelyhigh torque by controlling or changing a speed ratio of thetransmission. When the engine is operated in such condition, i.e., in alow-speed high-torque mode, the ignition period or cycle time of theengine tends to be long and consequently the engine suffers increasedperiodic variations in its output torque, whereby the drivability of thevehicle is degraded due to vibrations and noises which are caused by theperiodic variations of the engine torque. In such arrangement, theaverage output torque of the engine is sufficient to drive the vehicle.However, there remains a problem of drivability when the engine is runat speeds in a relatively low range.

In the meantime, an electromagnetic clutch using a mass of magneticpowder was developed, as disclosed in Japanese Patent Application whichwas laid open in 1983 under Publication No. 58-657. Such anelectromagnetic clutch is disposed between a vehicle engine and atransmission to transmit an output of the engine to the transmission.The clutch is controlled so that a rotating speed of its output shaftwhose torque is transmitted to the transmission is lower than a rotatingspeed of its input shaft by a predetermined amount which is greater thana magnitude of variation in the rotating speed of the input shaft(variation from the average speed) which is caused by periodicvariations of the torque of the input shaft. Namely, a slight amount ofslip which gives a speed differential between the input and outputshafts, is positively given to the electromagnetic clutch, so that avariation in output torque of the engine may be absorbed or accommodatedby the clutch, in order to permit the engine to be operated in alow-speed high-torque mode with a high fuel economy, while maintaining ahigh level of drivability of the vehicle.

In the above-described arrangement for controlling the electromagneticclutch, however, it is difficult for various reasons, to achieve anintricate control of an amount of slippage between the input and outputshafts of the clutch, for absorbing the torque variation of the enginewhile minimizing the transmission power loss within the clutch.

Stated in greater detail, the magnitude of the engine torque variationvaries depending upon current levels of speed and torque, and otherspecific operating conditions of the engine. Consequently, the controlto maintain a constant amount of slip of the electromagnetic clutch(constant speed difference between its input and output shafts) does notmake it possible to simultaneously accomplish the minimization of thepower loss and the absorption of the engine torque variation, in theengine range of the varying operation conditions of the engine. With aconstant amount of slip, the clutch slips too much and the fuel economyof the engine is lowered when the engine is operated with a relativelysmall level of variation in its output torque. If the amount of slip isreduced, on the contrary, the clutch is not able to sufficiently absorbthe engine torque variation and the drivability of the vehicle islowered when the engine is operated with a relatively large torque.

To solve the above inconveniences, it is considered to control thetransmission torque of the electromagnetic clutch so that the amount ofslip of the clutch is varied based on the detected output conditions ofthe engine. In this case, the amount of slip of the clutch may beoptimized to the varying magnitude of variation in the engine torque.However, this method is available only where the output characteristicsof the engine are unchanged. That is, it is impossible to give theclutch an optimum amount of slip that meets the varying magnitude of theengine torque, when the clutch is used for different types of engines orwhen the same engine is subject to changes in operating conditions suchas atmospheric pressure, temperature of cooling water, relative humidityof the atmosphere, amount of carbon deposit in combustion chambers,etc., or changes in output characteristics.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a methodand an apparatus for controlling an electromagnetic clutch connected toa vehicle engine, wherein a variation in output torque of the engine issuitably absorbed by the electromagnetic clutch for maximum drivabilityof the vehicle, while a transmission power loss within the clutch isheld to a minimum for maximum fuel economy of the engine.

According to one aspect of the present invention, there is provided amethod of controlling an electromagnetic clutch incorporated in a powertransmission system of a vehicle to transmit an output of an engine todrive wheels of the vehicle, so as to absorb a variation in outputtorque of the engine, wherein an engaging action of the electromagneticclutch is controlled to adjust a clutch torque transmitted by theclutch, based on whether a variation in rotating speed or torque of anoutput shaft of the clutch is synchronized with an ignition period ofthe engine.

According to anothe aspect of the invention, the above-described methodmay be suitably practiced by an apparatus wherein the engaging action ofthe clutch is controlled such that the clutch torque is varied as afunction of an amount of electric current to be applied to the clutch,the apparatus comprising: detecting means for detecting a variation inrotating speed or torque of an output shaft of the clutch; and currentadjusting means for controlling the amount of electric current to beapplied to the clutch, so that the amount of electric current isdecreased when the variation in rotating speed or torque of the outputshaft is detected by the detecting means, and increased when thevariation in rotating speed or torque of the output shaft is notdetected by the detecting means.

In the method and apparatus as described above, the engaging action ofthe elctromagnetic clutch and therefore the clutch torque transmitted bythe clutch are controlled depending upon whether the waveformrepresentative of variations in rotating speed or torque of the outputshaft of the clutch is in timed relation with the ignition period of theengine. Accordingly, the electromagnetic clutch is given an optimumamount of slip, irrespective of specific output conditions of the enginesuch as the engine speed and torque, irrespective of operatingconditions of the engine such as atmospheric pressure, temperature ofcooling water, relaive humidity of the atmosphere, and amount of carbondeposit in combustion chambers, irrespective of changing outputcharacteristics of the engine, and irrespective of specific types of theengine.

In a method according to a further aspect of the invention, a frequencyof variations in rotating speed or torque of the output shaft of theelectromagnetic clutch is compared with an ignition frequency of theengine, and the engaging action of the clutch is controlled to adjustthe clutch torque, based on whether the frequency of variations inrotating speed or torque of the output shaft is coincident with theignition frequency.

According to a still further aspect of the invention, the above methodmay be suitably practiced by an apparatus wherein the clutch torque isvaried as a function of an amount of electric current to be applied tothe clutch, the apparatus comprising: first detecting means fordetecting the ignition frequency of the engine; second detecting meansfor detecting the frequency of variations in rotating speed or torque ofthe output shaft of the clutch; and current adjusting means forcontrolling the amount of electric current, such that the amount ofelectric current is reduced when the detected frequency of variations iscoincident with the detected ignition frequency, and increased when thedetected frequency of variation is not coincident with the detectedignition frequency.

In the above-described method and apparatus, the clutch torque isreduced when the detected frequency of speed or torque variations in theoutput shaft is coincident with the detected ignition frequency, whilethe clutch torque is increased when the detected frequency of speed ortorque variations is not coincident with the detected ignitionfrequency. Hence, the clutch torque is controlled so that the timedrelation between the engine combustions and the speed or torquevariations in the output shaft of the clutch is slightly lost.Accordingly, the amount of slip in the electromagnetic clutch is kept atan optimum level suitable for absorbing the engine torque variation.

It is noted that the speed variations and torque variations of theoutput shaft of the clutch both take place in timed relation with theignition timing of the engine. Although the speed variations and thetorque variation are different in phase, both variations are representedby the same waveform. Therefore, the speed variations and the torquevariations, or the speed variation frequency and the torque variationfrequency, may be interchangeably used as variables which are comparedwith the ignition period or cycle time of the engine.

In accordance with another aspect of the invention, there is provided amethod of controlling an electromagnetic clutch incorporated in a powertransmission system of a vehicle to transmit an output of an engine todrive wheels of the vehicle, so as to absorb a variation in outputtorque of the engine, wherein the number of ignitions per unit time ofthe engine is compared with the number of variations per unit time intorque of an output shaft of the clutch, and an engaging action of theclutch is controlled to adjust a clutch torque transmitted by theclutch, such that a difference between the number of ignitions of theengine and the number of variations in torque of the output shaftcoincides with a predetermined reference value.

In accordance with a yet another aspect of the invention, theabove-described method may be suitably practiced by an apparatus whereinthe clutch torque is varied as a function of an amount of electriccurrent to be applied to the clutch, the apparatus comprising: firstdetecting means for detecting the number of ignitions per unit time ofthe engine; second detecting means for detecting the number ofvariations per unit time in torque of the output shaft of the clutch;and current adjusting means for controlling the amount of electriccurrent such that a difference between the detected number of ignitionsof the engine and the detected numbers of variations in torque of theoutput shaft coincides with a predetermined reference value.

In the method and apparatus described above, the clutch torque iscontrolled so that the difference between the detected number of theengine ignitions per unit time and the detected number of torquevariations per unit time of the output shaft of the clutch is equal tothe predetermined reference value. In this arrangement, the variation inoutput torque of the engine may be suitably absorbed or accommodated bythe electromagnetic clutch while the transmission power loss of theclutch is held at a minimum level. Accordingly, the engine may beoperated with a high fuel economy, and a considerably high degree ofdrivability is maintained even when the engine is operated with acomparatively large variation in its output torque.

The above-indicated predetermined reference value is determined so as topermit maximum absorption of the output torque variation of the engine,with substantially no power loss within the electromagnetic clutch.Consequently, the variation in the output torque of the engine may besuitably absorbed by the clutch, irrespective of the varying operatingconditions of the engine, and irrespective of the specific type of theengine. The reference value may be adjusted based on the current speedof the engine.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other objects, features and advantages of the presentinvention will be better understood from reading the following detaileddescription of preferred embodiments of the invention, when consideredin conjunction with the accompanying drawing, in which:

FIG. 1 is a block diagram of one embodiment of a control apparatus ofthe invention;

FIG. 2 is an elevational view in cross section of an electromagneticclutch which is controlled by the control apparatus of FIG. 1;

FIG. 3 is a graphical representation showing an example of powertransmission characteristics of the electromagnetic clutch of FIG. 2;

FIGS. 4, 5 and 6 are flow charts illustrating the operation of thecontrol apparatus of FIG. 1;

FIG. 7 is a view showing an irregular state of a waveform representativeof a variation in the rotating speed of the output shaft of theelectromagnetic clutch;

FIG. 8 is a graph showing successive pulse periods of rotation signalsof the output shaft of the electromagnetic clutch,

FIG. 9 is a graph showing a waveform representative of periodicvariations in the rotating speed of the output shaft of the clutch wherethe period of the speed variation waveform is coincident with anignition frequency of a vehicle engine;

FIG. 10 is a fragmentary flow chart showing another embodiment of theinvention;

FIG. 11 is a diagram corresponding to FIG. 1, showing a furtherembodiment of the invention;

FIG. 12 is a diagram also corresponding to FIG. 1, showing a stillfurther embodiment of the invention;

FIG. 13 is an elevational view in cross section of the electromagneticclutch of the embodiment of FIG. 12;

FIG. 14 is a graphical representation of power transmissioncharacteristics of the electromagnetic clutch of FIG. 13;

FIG. 15 is a view illustrating examples of various waveforms obtained inthe embodiment of FIG. 12;

FIG. 16 is a flow chart showing the operation of the embodiment of FIG.12;

FIG. 17 is a view illustrating a relation between a throttle openingangle and an operating speed of the engine of the embodiment of FIG. 12,while the vehicle is running; and

FIG. 18 is a graph illustrating the operating state of theelectromagnetic clutch of FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail, by refernece to theaccompanying drawings.

Referring first to FIG. 1, there is shown an electromagnetic clutch 14which connects a crankshaft 12 of a vehicle engine 10 and an input shaft18 of a transmission assembly 16. As described later in detail, theelectromagnetic clutch comprises a driving rotary member on its inputside, and a driven rotary member on its output side, so that theserotary members engaging with each other upon energization of a solenoid20 to produce a magnetic force. The produced magnetic force causes a gapbetween the rotary members to be filled with a mass of magnetic powder,whereby a torque corresponding to an amount of electric energizationcurrent of the solenoid 20 is transmitted from the driving rotary memberto the driven rotary member, according to given transmissioncharacteristics. The structural arrangement of the electromagneticclutch 14 is illustrated in FIG. 2, wherein an annular yoke 22 as thedriving rotary member is connected to the crankshaft 12 via an outermember 24. The solenoid 20 of annular shape is embedded in the annularyoke 22 and is supplied with an energization current via a slip ring 30.The slip ring 30 is secured to a first labyrinth member 28 which rotateswith the yoke 22. The driven rotary member in the form of a rotor 32 issupported by the first labyrinth member 28 via a bearing 34, such thatthe rotor 32 is concentric with, and rotatable relative to the yoke 22.The rotor 32 engages the end portion of the input shaft 18 by means of aspline. The first labyrinth member 28 is provided with an annular flange36, while the yoke 22 is provided, on the side of the crankshaft 12,with a second labyrinth member 38 which has another annular flangesimilar to the annular flange 36. The second labyrinth member 38cooperates with the annular flange 36 to define a substantially enclosedannular space which accommodates the magnetic powder mass. Uponenergization of the solenoid 20, the previously indicated gap betweenthe outer circumference of the rotor 32 and the inner circumference ofthe yoke 22 is filled with the magnetic powder mass, and the particlesof the magnetic powder are tightly bonded together so that the powdermass contacts the outer and inner surfaces of the rotor 32 and the yoke22. Thus, the rotary motion of the crankshaft 12 is transmitted to theinput shaft 18 of the transmission assembly 16 by the electromagneticclutch 14, such that a clutch torque transmitted by the clutch 14 to theinput shaft 18 corresponds to an amount of energization current appliedto the solenoid 20. As is apparent from the foregoing description, thecrankshaft 12 and the input shaft 18 act as input and output shafts ofthe electromagnetic clutch 14, respectively. In this connection, a curveof FIG. 3 represents an example of a relation between the clutch torquetransmitted by the clutch 14, and the amount of energization current ofthe solenoid 20.

Referring back to FIG. 1, the transmission assembly 16 comprises aplural-step transmission or a stepless transmission equipped with anauxiliary transmission, and a final drive assembly including adifferential unit. The transmission assembly 16 transmits a torque ofits input shaft 18 to drive wheels 42 of a vehicle through an axle 40.The engine 10 is provided with an ignitor 44 which generates an IGNITIONsignal SI corresponding to an ignition pulse aplied to produce a spartto ignite a fuel charge in the engine 10. The IGNITION signal SI isapplied to an I/F circuit 48 in a controller 46, so that the rotatingspeed of the crankshaft 12 is detected. To the input shaft 18 and axle40, there are fixed speed detector gears 50, 52, respectively, each ofwhich has a plurality of detector teeth on its outer circumference, forsensing the rotating speeds of the input shaft 18 and axle 40. Adjacentto these speed detector gears 50, 52, there are disposed speed sensors54, 56, respectively, which generate ROTATION signals SR1 and SR2 in theform of pulses whose frequencies correspond to the rotating speeds ofthe input shaft 18 and axle 40, respectively. The ROTATION signals SR1,SR2 are fed to the I/F circuit 48. The pulses of the ROTATION signalsSR1, SR2 are produced in response to passage of the detector teethacross the speed sensors 54, 56. Since the speed sensor 54 is assignedto detect not only the rotating speed of the input shaft 18, but alsovariation in the rotating speed, the corresponding speed detector gear50 should preferably have at least 10 detector teeth.

A clock generator 58 applies clock pulses of a relatively high frequencyto the I/F circuit 48. The I/F circuit 48 applies to a PERIOD counter 60the clock pulses whose numbers correspond to periods of the IGNITION andROTATION signals SI, SR1 and SR2. Based on the numbers of the receivedclock pulses, the PERIOD counter determines periods of the IGNITION andROTATION signals SI, SR1 and SR2.

The controller 46 is constituted by a so-called microcomputer whichcomprises a CPU 62, a ROM 64 and a RAM 66. The CPU 62 operates toprocess input data according to a program stored in the ROM 64, whileutilizing a temporary storage function of the RAM 66, and supplies acontrolled energization current to the solenoid 20 of theelectromagnetic clutch 14 via an output circuit 68. The output circuit68 includes a D/A converter and a voltage/current converter.

The operation of the present embodiment will be described.

In the instant embodiment, a speed data input routine of FIG. 5 forstoring speed data of the input shaft 18, and a memory switching routineof FIG. 4 for alternately selecting two memories (later describedbuffers A and B provided in the RAM 66) are executed as interruptionroutines while a clutch control main routine of FIG. 6 is interrupted.The speed data input routine is repeated at a relatively short intervalwhich is determined in view of a calculation time necessary for theclutch control main routine. Further, the memory switches routine isrepeated at an interval which is determined in view of the interval ofthe speed data input routine. Stated in more detail, the memoryswitching routine of FIG. 4 is effected in preference to the clutchcontrol main routine, for example, each time the IGNITION signal SIrepresentative of an ignition or combustion of the engine 10 isgenerated, or for every second IGNITION signal SI. In the memoryswitching routine, step SW 1 is executed to reverse the current contentof a flag F1 (which will be described), and step SW2 is carried out toset the content of a flag F2 (which will be described) to "1". The flagF1 serves to alternately select one of the buffers A and B in which thespeed data of the input shaft 18 is to be stored. The flag F2 indicatesa state in which a new or updated amount of control of the clutch torquehas not yet been determined based on a series of speed data alreadystored in the selected buffer A or B.

The speed data input routine of FIG. 5 is effected, for example, inresponse to the generation of each ROTATION signal SR1, and inpreference to the clutch control main routine. In this routine, step SS1is executed to read out the period "tn" of the ROTATION signal SR1 whichhas been determined by the PERIOD counter 60, and step SS2 is thenexecuted to store the period "tn" in the buffer A or B which is selectedby the flag F1. With this speed data input routine repeated in timedrelation with the ROTATION signals SR1, a series of period data "tn"(speed data) of the input shaft 18 are stored one after another in thebuffer A or B designated by the flag F1. The buffers A and B arealternately selected for a time span equal to a time interval betweentwo consecutive ignitions of the engine 10, with the flag F1 changedfrom one state to the other in step SW1 of the memory switching routine.In this time span, a relatively large number of the period data "tn" ofthe ROTATION signals SR1 are stored in the selected buffer A or B. Inthis connection, it is noted that the speed of the input shaft 81 aftereach ignition of the engine is raised to its highest level and loweredto its lowest level at which the next combustion takes place to raisethe speed to its highest level. Each cycle of storing the series of theperiod data "tn" in the selected buffer is started so that a period oftime during which the speed is lowered and raised past the lowest levelis wholly included in each storage cycle. In other words, the timing ofeach storage of the period data "tn" with respect to the ignition of theengine is determined so that the bottom portion of the waveform of thespeed variation period data "tn" of the input shaft 18 does not bridgetwo consecutive ignition cycles of the engine 10.

Referring next to FIG. 6, the clutch control routine will be described.Initially, step S1 is executed to store in the RAM 66 a period "tv" ofthe ROTATION signal SR2 determined by the PERIOD counter 60. Step S1 isfollowed by step S2 wherein a vehicle speed "v" is calculated based onthe read-out period "tv" and according to the following equation (1):

    v=1/tv.N×60 sec.×60 min.×2πr×10.sup.-3 km/h (1)

where,

N: Number of teeth of the speed detector gear 52

r: Radius of the drive wheels 42

In the next step S3, the CPU 62 checks the calculated vehicle speed "v"to judge whether the electromagnetic clutch 14 should be partiallyengaged, or should be controlled so as to absorb a variation in outputtorque of the engine 10 with substantially no slip. For example, wherethe vehicle speed "v" is lower than 10 km/h, the CPU 62 goes to step S4to control the electromagnetic clutch 14 for partial engagement thereof.In the partial engagement mode, a control amount "Vcl" (e.g., a controlvoltage to be applied to the voltage/current converter in the outputcircuit 68) corresponding to the energization current of the solenoid 20is determined by the following equation (2):

    Vcl=(Ne-Nidl)×K                                      (2)

where,

Ne: Speed of the engine 10

Nidl: Idling speed of the engine 10

K: Gain, i.e., constant or function of throttle opening angle or targetspeed of the engine 10.

Where the CPU 62 judges in step S3 that the vehicle speed "v" is equalto or higher than 10 km/h, step S3 is followed by step S5 to check ifthe content of the flag F2 is "1" or not. If the content of the flag F2is not "1", that is, if the storage of the period data "tn" into theselected buffer A or B has not been completed, the CPU 62 goes to stepS17 wherein the control amount "Vcl" determined in the preceding cycleis kept unchanged and continuously applied to the solenoid 20 of theclutch 14. In the case where the content of the flag F2 is "1", namely,where the storage of the period data "tn" in the buffer A or B has beencompleted but a control amount for the solenoid 20 has not beendetermined based on the period data "tn", step S5 is followed by step S6to inhibit the execution of the previously indicated interruptionroutines, i.e., memory switching routine of FIG. 4 and speed data inputroutine of FIG. 5, until step S10 is executed. During this period ofinhibition, the CPU 62 executes steps S7 through S9. In step S7, the CPU62 judges whether the content of the flag F1 is "1" or not. If thecontent of the flag F1 is "0" and the buffer A is currently selected forstorage of the period data "tn" therein, step S7 is followed by step S8wherein the period data "tn" which has been stored in the previouslyselected buffer B are transferred to a buffer C. If the content of theflag F1 is "1", the CPU 62 goes to step S9 in which the period data "tn"already stored in the buffer A are transferred to the buffer C.

In step S11, the CPU 62 selects the maximum period "tn max" from amongthe series of the period data "tn" stored in the buffer C. Then, stepS11 is followed by step S12 to check if the period "tn max" is greaterthan the preceding period "tn-1" and to check if the period "tn-1" isgreater than the preceding period "tn-2". Namely, step S12 is executedto check whether the values of the successive periods "tn" increaseprogressively in the order of "tn-2", "tn-1" and "tn max". If the resultof the checking in step S12 is affirmative, the CPU 62 goes to step S13to check if the period "tn max" is greater than the following period"tn+1" and to check if the period "tn+1" is greater than the followingperiod "tn+2". In other words, step S13 is executed to check whether thevalues of the successive periods "tn" decrease progressively in theorder of "tn max", "tn+1" and "tn+2". In the event that the result ofthe checking in step S12 or in step S13 is negative, it means that therotating speed of the output shaft of the electromagnetic clutch 14,i.e., of the input shaft 18 of the transmission assembly 16, is notvaried in timed relation with the ignition timing or period of theengine 10. That is, the waveform representative of the speed variationof the input shaft 18 is not synchronized with the generation of theIGNITION signals SI or combustion of the engine 10, as indicated in FIG.7. In this case, therefore, the CPU 62 does to step S14 wherein apredetermined small value ΔV is added to the preceding control value"Vcl" to increase and update the control voltage to adjust theenergization current of the solenoid 20 of the clutch 14. Stated morespecifically, when the speed variation waveform of the input shaft 18 isnot synchronized with the ignition timing of the engine 10, it indicatesthat the electromagnetic clutch 14 currently undergoes an excessiveamount of slip and suffers an unnecessarily large power loss. In thiscondition, therefore, the energization current of the solenoid 20 isincreased to reduce the amount of slip of the electromagnetic clutch 14.

In the case where the results of the checking in steps S12 and S13 areboth affirmative, namely, if the periods "tn" prior to the maximumperiod "tn max" increase progressively to the maximum level "tn max",while the periods "tn" following the period "tn max" decreaseprogressively, as indicated in FIG. 8, then the CPU 62 judges that thespeed variation waveform of the input shaft 18 is synchronized with thetiming in which the IGNITION signals SI are generated, as indicated inFIG. 9. In this condition, the CPU 62 goes to step S15 in which apredetermined value ΔV is subtracted from the control amount "Vcl" whichwas determined in the preceding cycle, whereby the control amount "Vcl"is updated. Described in more detail, where the speed variation waveformof the input shaft 18 is synchronized with the IGNITION signals SI andtherefore with the combustion timing of the engine 10, it means that theelectromagnetic clutch 14 tends to undergo a comparatively small amountof slip. Accordingly, the electromagnetic clutch 14 is in a state inwhich it is impossible to sufficiently absorb a variation in the outputtorque of the engine 10. Hence, the control voltage for the solenoid 20is reduced to increase the amount of slip in the clutch 14. It is notedthat the period "tn max" of FIG. 8 of the ROTATION signal SR1corresponds to the bottom of the speed variation waveform of FIG. 9 ofthe input shaft 18.

In the present embodiment, means for executing steps S11 through S13serves as means for detecting a variation in the rotating speed of theinput shaft 18, i.e., of the output shaft of the electromagnetic clutch14. Further, means for executing steps S14 and S15 serves as means forcontrolling a magnitude or amount of electric current to be applied toenergize the solenoid 20, in order to adjust the engaging condition ofthe electromagnetic clutch 14, i.e., a torque transmitted by the clutch14.

Then, the CPU 62 goes to step S16 to set the content of the flag F2 to"0", and to the previously indicated step S17 in which the updatedcontrol amount "Vcl" is applied to the output circuit 68. By repeatingthe above-described clutch control routine, the energization currentapplied to the solenoid 20 is controlled so that the speed variation ofthe input shaft 18 of the transmission assembly 16 takes a waveformwhich is intermediate between those of FIGS. 7 and 9. With the solenoidenergization current thus controlled, the engaging condition of theelectromagnetic clutch 14 is regulated so as to maintain an optimumamount of slip that allows sufficient absorption of a variation in theoutput torque of the engine 10, while minimizing the transmission powerloss.

As described hitherto, the electromagnetic clutch 14 is controlled suchthat the torque to be transmitted by the clutch 14 is adjusted tomaintain a condition wherein the synchronization or timed relation ofthe speed variation waveform with the combustion or ignition timing orperiod of the engine 10 is slightly lost. Stated the other way, theengaging condition and consequently the amount of slip of theelectromagnetic clutch 14 is controlled through adjustment of theenergization current of the solenoid 20, so that the speed variationperiod of the output shaft of the clutch 14 is neither strictlycoincident with the ignition period of the engine 10, nor excessivelydeviated from the ignition period. As a result, the instant arrangementpermits effective absorption of a variation in the output torque of theengine 10, and at the same time restrains the power loss within theclutch 14, irrespective of an actual output condition of the engine 10,specific type of the engine 10 (specific output torque characteristicsthereof), changes in the operating environments of the engine 10 such asatmospheric pressure, cooling water temperature, relative humidity ofthe atmosphere, amount of carbon accumulation in the combustionchambers, etc. Consequently, a common control program may be utilizedfor economical control of the transmission torque of the electromagneticclutch 14, for different types of the engine 10, and for various andchanging operating conditions of the engine.

While the foregoing embodiment uses the periods "tn" of the ROTATIONsignals SR1 to obtain the speed variation waveform of the input shaft18, it is also possible to detect a speed variation frequency of theinput shaft 18 and control the electromagnetic clutch 14 so that thecorresponding relation of this speed variation frequency with theignition frequency of the engine 10 is slightly lost.

Referring to FIG. 10, there will be described the above-indicatedmodified embodiment wherein the speed variation frequency of the inputshaft 18 is compared with the ignition frequency of the engine 10. Inthe interest of brevity and simplification, the same referencecharacters as used in the preceding embodiment will be used, and therepeated description of the corresponding elements will not be provided.

In this embodiment, steps S20 and S21 of FIG. 10 are executed in placeof steps S11 through S13 of the preceding embodiment. Described morespecifically, step S20 is executed to calculate a frequency "f" of thespeed variation of the input shaft 18, based on the series of the perioddata "tn" of the ROTATION signals SR1 which have been transferred to thebuffer C. Step S20 is followed by step S21 in which the calculated speedvariation frequency "f" is compared with an ignition frequency "fe" ofthe engine 10. In the case where these two frequencies "f" and "fe"coincide with each other, step S21 is followed by the previouslydescribed step S15 wherein the control amount "Vcl" is reduced. However,if the speed variation frequency "f" is not coincident with the ignitionfrequency "fe" of the engine 10, step S21 is followed by the previouslydescribed step S14 wherein the control amount "Vcl" is increased toreduce the amount of slip of the clutch 14. In this embodiment, meansfor executing step S20 serves as means for detecting the speed variationfrequency "f" of the input shaft 18, and means for executing steps S21,S14 and S15 serves as means for controlling the magnitude of theelectric current applied to energize the solenoid 20. While the amountof slip of the electromagnetic clutch 14 is judged by means ofcomparison of the speed variation frequency "f" with the ignitionfrequency "fe", it is also possible to compare the speed variationfrequency "f" with a combustion frequency of the engine 10, to check theclutch 14 for optimum amount of slip. In either case, substantially thesame results as obtained in the preceding embodiment are provided.

In the preceding embodiments, the control of the clutch 14 formaintaining an optimum slip is effected based on the speed variationwaveform (period data "tn") or the speed variation frequency "f" of theinput shaft 18.

However, it is possible to detect a torque variation waveform offrequency "ft" of the input shaft 18, and compare this waveform offrequency "ft" with the ignition period or frequency "fe" of the engine10. Although this torque variation waveform is slightly shifted in phasefrom the speed variation waveform of FIG. 9, the former is similar inform to the latter. Accordingly, the above modification to use thetorque variation waveform or frequency "f" provides essentially the sameresults as obtained in the preceding embodiments. As shown in FIG. 11,this modified arrangement employs a torque sensor 70 to detect an actualtorque of the input shaft 18, and an A/D converter 72 to convert theoutput signal of the torque sensor 70 into a digital value. In the casewhere the torque variation waveform is used, the operation to controlthe electromagnetic clutch 14 is performed through execution of stepssimilar to those shown in FIGS. 4, 5 and 6. In this case, however, stepsSS3 and SS4 of FIG. 5 are replaced by the steps in which the buffers Aand B store torque variation data of the input shaft 18, rather than thespeed variation period data "tn" of the ROTATION signal SR1. Moreprecisely, the buffers A and B store data representative of values ofthe torque of the input shaft 18 upon generation of each ROTATION signalSR1. In this case, means for executing the steps corresponding to stepsS11 through S13 of FIG. 6 serves as means for detecting the torquevariation of the input shaft 18.

In the case where the torque variation frequency "f" is used, stepssimilar to the steps of FIG. 10 are executed. In this case, however,step S20 of FIG. 10 is replaced by a step in which the torque variationfrequency "ft" of the input shaft 18 is calculated. Further step S21 ofFIG. 10 is replaced by a step in which the calculated torque variationfrequency "ft" is compared with the ignition frequency "fe" of theengine 10. In this case, therefore, means for executing the stepscorresponding to step S20 of FIG. 10 serves as means for detecting thetorque variation frequency "f" of the input shaft 18, and means forexecuting the steps corresponding to the steps S21, S14 and S15 servesas means for controlling the magnitude of the energization current ofthe solenoid 20.

As described hitherto, the solenoid 20 of the electromagnetic clutch 14is controlled based on the speed variation period "tn" or torquevariation period of the input shaft 18 as compared with the ignitionperiod of the engine 10, or based on the speed variation frequency "f"or torque variation frequency "ft" of the input shaft 18 as comparedwith the ignition frequency "fe" of the engine 10. Since the frequencyis inversely proportional to the period, the control of the clutch 14based on the frequency provides substantially the same results as thecontrol based on the period.

A still further embodiment of the invention will be described.

FIG. 12 shows a control device for controlling an electromagnetic clutch110 which connects a vehicle engine 112 to a transmission assembly 118which includes a transmission and a final drive unit incorporating adifferential device. Power of the engine 112 which is transmitted to thetransmission assembly 118 via the clutch 110 is transmitted to drivewheels 120. As shown in detail in FIG. 13, the electromagnetic clutch110 comprises a driving rotary member 124 connected to a crankshaft 122of the engine 112, a driven rotary member in the form of a rotor 128connected to an output shaft 126, and a solenoid 130 for changing thecondition of a mass of magnetic powder accommodated between the drivingrotary member 124 and the rotor 128, so that the torque transmitted bythe clutch 110 is adjusted. The crankshaft 122 serves as an input shaftof the electromagnetic clutch 110. The driving rotary member 124comprises an outer member 132 fixed to the crankshaft 122, and anannular yoke 134 secured to the inner surface of the outer member 132.The solenoid 130 of annular shape is embedded in the annular yoke 134and is supplied with an energization current via a brush (not shown) ofa slip ring 136 which is rotated with the yoke 134. Within the yoke 134,the rotor 128 is rotatably supported by a first labyrinth member 140 viaa bearing 138. The first labyrinth member 140 is secured to one end faceof the yoke 134 on the side of the output shaft 126, and is providedwith an annular flange 142. This annular flange 142, and a secondlabyrinth member 144 secured to the other end face of the yoke 134,cooperate with each other to define a substantially enclosed annularspace which accommodates the magnetic powder mass. Upon energization ofthe solenoid 130, a magnetic field is produced between the rotor 128 andthe yoke 134, whereby an annular gap between the outer circumference ofthe rotor 128 and the inner circumference of the yoke 134 is filled withthe magnetic powder mass. Thus, the rotary motion of the crankshaft 122is transmitted to the output shaft 126, according to the current-torquecharacteristics as shown in FIG. 14. The output shaft 126 is coupled atits end to a hub 146 by means of a spline. The hub 146 is connected tothe rotor 128 through a damper 148 which is provided to absorb anengagement shock of the clutch 110.

Referring back to FIG. 12, the engine 112 is provided with an ignitor156 which produces an IGNITION signal SI corresponding to an ignitionpulse applied to the engine 112. The IGNITION signal SI is applied to awaveform shaping circuit 158 in a controller 152. The output shaft 126of the electromagnetic clutch 110 is provided with a torque sensor 160which generates a TORQUE signal ST representative of a torque of theoutput shaft 126. The TORQUE signal ST is applied to an AC coupling 154.To detect a vehicle speed, a speed detector disk (not shown) is disposedon a shaft which is rotated with the drive wheels 120. This speeddetector disk has a multiplicity of detector teeth on its circumference.Adjacent to the detector disk, there is disposed a speed sensor 162which detects the passage of the detector teeth of the disk, andproduces a ROTATION signal SR in the form of pulses whose numbercorresponds to the number of the detected detector teeth of the disk.The ROTATION signal SR from the speed sensor 162 is fed to the waveformshaping circuit 158.

The AC coupling 154 transfers varying components T of the TORQUE signalST from the torque sensor 160 to the waveform shaping circuit 158. ATORQUE waveform A shown in FIG. 15 represents variations T in the torqueof the output shaft 126 of the electromagnetic clutch 110 while thecrankshaft 122 is connected to the output shaft 126 without a slip inthe electromagnetic clutch 110. A TORQUE waveform B shows variation T inthe torque of the output shaft 126 when the clutch 110 begins to slip.As described above, the waveform shaping circuit 158 receives theIGNITION signal SI (indicated at the top of FIG. 15) from the ignitor156, the varying components T of the TORQUE signal ST from the ACcoupling 154, and the ROTATION signal ST from the speed sensor 162.These signals are shaped into rectangular pulse signals by the waveformshaping circuit 158, and the shaped pulse signals are applied to aprogrammable counter 150. For reliable detection of the number ofvariations in the torque value of the output shaft 126, a thresholdlevel SL is provided for the TORQUE signal ST, as indicated in FIG. 15.This threshold level SL is set to be sufficiently higher than the noiselevel, and lower than the peak value of the torque variation waveformwhich corresponds to the firing of the engine 112. When the torquevariation waveform exceeds the preset threshold level SL, a rectangularpulse indicative of a variation in the torque is generated by thewaveform shaping circuit 158. A PULSE waveform PB at the bottom of FIG.15 is a waveform which is obtained as a result of shaping the TORQUEwaveform B. Thus, the waveform shaping circuit 158 serves as means forsensing a variation in the torque of the output shaft 126 of the clutch110. The programmable counter 150 counts the numbers of clock pulseswhich are received from a clock generator 170 for each time interval ofadjacent two pulse signals of the IGNITION signal SI and of the ROTATIONsignal SR. Based on the counted numbers of the clock pulses, theprogrammable counter 150 provides IGNITION period data "tig" of theIGNITION signal SI, and ROTATION period data "tout" of the ROTATIONsignal SR. The programmable counter 150 comprises an IGNITION counterfor counting the number of ignitions of the engine 112 to provideIGNITION number data "Tig", and a TORQUE VARIATION counter for countingthe number of variations in the torque of the output shaft 126, toprovide TORQUE VARIATION number data "Tq". The IGNITION number data"Tig" is prepared based on the number of ignitions which is counted fora sampling time determined by a CPU 164 (which will be described). TheTORQUE VARIATION number data "Tq" is prepared by counting the number ofpulses of the PULSE waveform PB which are produced by the waveformshaping circuit 158 for the sampling time determined by the CPU 164.Thus, the programmable counter 150 serves as means for counting thenumber of variations in the torque of the output shaft 126 of theelectromagnetic clutch 110.

The controller 152 is constituted by a so-called microcomputer, whichserves as means for controlling an electric current applied to thesolenoid 130 of the electromagnetic clutch 110. The controller 152comprises the previously indicated programmable counter 150 and CPU 164,and a ROM 166, a RAM 168, and an output circuit 172, which are connectedto each other by a data bus line. The CPU 164 operates to process theinput data from the programmable counter 150, according to a programstored in the ROM 166, while utilizing a temporary storage function ofthe RAM 168. The CPU 164 supplies via the output circuit 172 acontrolled energization current "Icl" to the solenoid 130 of theelectromagnetic clutch 110. The output circuit 172 includes a D/Aconverter and an amplifier.

The operation of the present embodiment will be described, by referenceto the flow chart of FIG. 16 which depicts essential events of operationunder control of the CPU 164.

Initially, the CPU 164 goes to step SA1 to store the ROTATION perioddata "tout" of the ROTATION signal SR, and the IGNITION period data"tig" of the engine 112. Step SA1 is followed by Step SA2 in which avehicle speed V is calculated according to the following equation (3):

    V(km/h)=1/tout×1/n×60 sec.×60 min.×2π×r.sub.D +10.sup.-3                 (3)

where,

r_(D) : Radius of the drive wheels 120

n: Number of detector teeth of the detector disk Subsequently, the CPU164 goes to step SA3 to check if the calculated vehicle speed V isgreater than or equal to a predetermined value Vα or not. This value Vαis set, for example, at about 25 Km/h, and used as a boundary belowwhich the electromagnetic clutch 110 is controlled in the partialengagement mode. In the case where the vehicle speed V is lower than thepredetermined value Vα, the CPU 164 goes to step SA4 to reset a timercounter T, and then to step SA5 to calculate the rotating speed of theinput shaft of the clutch 110, i.e., speed "Ne" of the engine 112,according to the following equation (4):

    Ne(r.p.m.)=1/tig×1/2×60 sec.                   (4)

where, the engine 112 has four cylinders, and two ignition pulses areapplied for each revolution of the engine 112.

Also in step SA5, the CPU 164 determines a control voltage "Vcl"according to the following equation (5):

    Vcl=K(Ne-Nidl)                                             (5)

where,

Nidl: idling speed of the engine 112

K: Control factor determined by throttle opening angle, etc.

Step SA5 is folowed by step SA6 in which the determined control voltage"Vcl" is applied to the output circuit 172, whereby the energizationcurrent "Icl" is supplied from the output circuit 172 to the solenoid130 of the electromagnetic clutch 110.

In the case where the checking in step SA3 reveals that the calculatedvehicle speed V is greater than or equal to the predetermined value Vα,the CPU 164 executes step SA7 and the subsequent steps in order tocontrol the electromagnetic clutch 110 such that a variation in theoutput torque of the engine 112 is absorbed by the clutch 110. The timercounter T is incremented in step SA7, and the content of the counter Tis compared with a predetermined value Tα in step SA8. The predeterminedvalue Tα determines the previously indicated sampling time during whichthe IGNITION and TORQUE VARIATION counters of the programmable counter150 count the number of variations in the torque of the output shaft126, and the number of ignitions of the engine 112, respectively. Whenthe current content of the timer counter T has not yet reached thepredetermined value Tα, the CPU 164 skips steps SA9-SA13, and goes tostep SA6. In the case where the content of the counter T has reached thepredetermined value Tα, step SA8 is followed by step SA9 wherein theIGNITION and TORQUE VARIATION counters (which have been turned on instep SA13) are turned off. More specifically, step SA9 is executed toterminate the sampling time during which the numbers of ignitions andtorque variations are counted, and to store the IGNITION number data"Tig" and the TORQUE VARIATION number data "Tq" which are prepared basedon the contents of the IGNITION and TORQUE VARIATION counters of theprogrammable counter 150.

In the next step SA10, the CPU 164 checks if the electromagnetic clutch110 is placed in a condition in which it is possible to sufficientlyabsorb a variation in the output torque of the engine 112 withsubstantially no power loss. Described in more detail, the step SA10 isexecuted to check whether an absolute value of a difference between theIGNITION and TORQUE VARIATION number data "Tig" and "Tq" (stored in stepSA9) is greater than or equal to a predetermined reference value ΔT*.This reference vlaue ΔT* is determined by multiplying the IGNITIONnumber data "Tig" by a constant k which is smaller than "1" (ΔT*=Tig×k).If the absolute value of the above difference is less than the referencevalue ΔT*, it means that the amount of slip in the clutch 110 is notsufficient to allow the absorption of a variation in the engine 112. Inthis instant, the CPU 164 goes to step SA11 wherein the control voltage"Vcl" is updated by subtracting a predetermined value ΔV from the lastvalue "Vcl.sup.(n-1) ". If the absolute value of the above difference isgreater than or equal to the predetermined reference value ΔT*, itindicates that the amount of slip in the clutch 110 is sufficient toabsorb the engine torque variation, but is too large to preventexcessive power loss in the clutch 100. In this condition, the CPU 164goes to step SA12 in which the control voltage "VCl" is updated to addthe predetermined value ΔV to the last value "Vcl.sup.(n-1) ".

After the control voltage "Vcl" has been determined as described above,step SA13 is executed to reset and then turn on the IGNITION and TORQUEVARIATION counters of the programmable counter 150. Step SA14 isfollowed by step SA14 to reset the timer counter T. It is noted that thetimer counter T is constituted by the CPU 164 which executes a programfor counting the predetermined sampling time, and is not shown in thedrawing.

For higher fuel economy, it is desired that the engine 112 be operatedat a relatively low speed and with a relatively high torque, as shown inthe hatched area in FIG. 17. In a conventional arrangement wherein theengine power is faithfully transmitted to the vehicle drive wheels, theoperation of the engine in the low-speed high-torque range of FIG. 17will cause comparatively large variations in the output torque of theengine and therefore lead to reduced drivability or degraded drivingcomfort. Thus, the increase in the fuel economy of the engine may beenjoyed in the low-speed high-torque range, but at the sacrifice of thevehicle drivability.

In the meantime, the electromagnetic clutch 110 exhibits thecurrent-torque characteristics as shown in FIG. 14. Because of thischaracteristics of the clutch 110, the clutch 110 has an increasingamount of slip between its input shaft (crankshaft 122) and its outputshaft 126, as the energization current Icl to be applied to its solenoid130 is reducd as shown in FIG. 18. As the amount of slip in the clutch110 is increased, the magnitude of a variation in the transmissiontorque tends to be gradually decreased as indicated in the middle ofFIG. 18. While the energization current Icl is relatively large, therotating speeds "Nin" and "Nout" of the input and output shafts 122, 126of the clutch 110 are varied periodically in synchronization with eachother, as indicated at the bottom of FIG. 18. As the energizationcurrent is reduced, the force of coupling between the input and outputshafts 122, 126 is gradually reduced and the clutch 110 begins toundergo a slip. With the current falling below a given limit, theperiodic variation in the output speed "Nout" of the output shaft 126disappears. However, the input speed "Nin" of the input shaft(crankshaft) 122 is increased relative to the output speed "Nout", whilecontinuing the periodic variation. Therefore, it will be understood thatthere exits the most favorable condition, in the vicinity of a brokenline K indicated in FIG. 18, in which the variation in the torque of theengine 112 is effectively absorbed by the clutch 110 and is nottransferred to the output shaft 126, while at the same time the clutch110 undergoes substantially no slip (between the input and output shafts122, 126).

The present inventors found a phenomenon that, under the above-indicatedcondition, a difference between the number of combustions of the engine112 per unit time, i.e., the number of ignitions "Tig" per unit time andthe number of torque vairations "Tq" of the output shaft 126 per unittime is constant provided the other conditions such as the speed of theengine 112 are constant. Accordingly, the transmission torque of theelectromagnetic clutch 110 is controlled so that the difference betweenthe number of ignitions "Tig" and the number of torque variations "Tq"coincides with the predetermined difference ΔT* which is used in stepSA10. In other words, the energization current Icl of the solenoid 130is controlled so as to establish the most favorable condition asindicated by the broken line K in FIG. 18.

To control the energization current Icl, the control voltage Vcl to beapplied to the output circuit 172 is determined in steps SA10-SA12. Withthe thus determined control voltage Vc1, the energization current Icl tobe supplied from the output circuit 172 to the solenoid 130 is adjustedsuch that the difference between the IGNITION and TORQUE VARIATIONnumber data "Tig" and "Tq" is equal to the predetermined difference ΔT*.In this way, the electromagnetic clutch 110 is held in the mostfavorable condition K or its vicinity. In this condition, the periodicvariation in the output torque of the engine 112 is suitably absorbed oraccommodated by the electromagnetic clutch 110, yet with minimum powerloss, even while the engine 112 is operated in the range shown in FIG.17. Hence, the vehicle equipped with an electromagnetic clutchcontrolled by the instant control device provides relatively highdrivability as well as relatively high fuel economy.

Although the reference value ΔT* used in the instant embodiment isdetermined by multiplying the IGNITION number data "Tig" by a suitablefactor "k", the reference value may be constant, or may be determined bythe output torque of the engine as well as by the ignition number "Tig"(engine speed).

The programmable counter 150 of FIG. 12 may be replaced by a programwhich is stored in the ROM 166 and executed by the CPU 164 to attain thesame function as performed by the counter 150.

While the present invention has been described in its preferredembodiments, it is to be understood that the invention is not confinedto the precise disclosure contained herein, but may be otherwiseembodied with various changes, modifications and improvements which mayoccur to those skilled in the art, without departing from the spirit andscope of the invention defined in the appended claims.

What is claimed is:
 1. A method of controlling .[.an electromagnetic.]..Iadd.a .Iaddend.clutch .Iadd.means .Iaddend.incorporated in a powertransmission system of a vehicle to transmit an output of an engine todrive wheels of the vehicle, so as to absorb a variation in outputtorque of the engine, .Iadd.a clutch torque transmitted by said clutchmeans being varied through an electromagnetic force, .Iaddend.saidmethod comprising the steps of:detecting a variation in rotating speedor torque of an output shaft of said .[.electromagnetic.]. clutch.Iadd.means.Iaddend.; and controlling an engaging action of said.[.electromagnetic.]. clutch .Iadd.means .Iaddend.to adjust .[.a.]..Iadd.said .Iaddend.clutch torque transmitted by said clutch.Iadd.means.Iaddend., based on whether the detected variation inrotating speed or torque of said output shaft of said.[.electromagnetic.]. clutch .Iadd.means .Iaddend.is synchronized withan ignition period of said engine.
 2. An apparatus for controlling .[.anelectromagnetic.]. .Iadd.a .Iaddend.clutch .Iadd.means having anelectromagnetic coil .Iaddend.incorporated in a power transmissionsystem of a vehicle to transmit an output of an engine to drive wheelsof the vehicle, so as to absorb a variation in output torque of theengine, an engaging action of said .[.electromagnetic.]. clutch.Iadd.means .Iaddend.being controlled to adjust a clutch torquetransmitted by said clutch .Iadd.means .Iaddend.such that said clutchtorque is varied as a function of an amount of electric current .Iadd.orvoltage .Iaddend.applied to the .[.electromagnetic.]. clutch.Iadd.means.Iaddend., said control apparatus comprising:detecting meansfor detecting a specific pattern of a variation in rotating speed of anoutput shaft of said .[.electromagnetic.]. clutch .Iadd.means.Iaddend.,the specific pattern being synchronized with an ignition period of saidengine; and current adjusting means for controlling said amount ofelectric current .Iadd.or voltage .Iaddend.so that the amount ofelectric current .Iadd.or voltage .Iaddend.is decreased when saidspecific pattern of variation in rotating speed of said output shaft isdetected by said detecting means, and increased when said specificpattern of variation is not detected by said detecting means.
 3. Anapparatus for controlling .[.an electromagnetic.]. .Iadd.a.Iaddend.clutch .Iadd.means having an electromagnetic coil.Iaddend.incorporated in a power transmission system of a vehicle totransmit an output of an engine to drive wheels of the vehicle, so as toabsorb a variation in output torque of the engine, an engaging action ofsaid .[.electromagnetic.]. clutch .Iadd.means .Iaddend.being controlledto adjust a clutch torque transmitted by said clutch .Iadd.means.Iaddend.such that said clutch torque is varied as a function of anamount of an electric current .Iadd.or voltage .Iaddend.applied to the.[.electromagnetic.]. clutch .Iadd.means.Iaddend., said controlapparatus comprising:detecting means for detecting a specific pattern ofvariation in torque of an output shaft of said .[.electromagnetic.].clutch .Iadd.means.Iaddend., the specific pattern being synchronizedwith an ignition period of said engine; and current adjusting means forcontrolling said amount of electric current .Iadd.or voltage .Iaddend.sothat the amount of electric current .Iadd.or voltage .Iaddend.isdecreased when said specific pattern of variation in torque of saidoutput shaft is detected by said detecting means, and increased whensaid specific pattern of variation is not detected by said detectingmeans.
 4. A method of controlling .[.an electromagnetic.]. .Iadd.a.Iaddend.clutch .Iadd.means .Iaddend.incorporated in a powertransmission system of a vehicle to transmit an output of an engine todrive wheels of the vehicle, so as to absorb a variation in outputtorque of said engine, .Iadd.a clutch torque transmitted by said clutchmeans being varied through an electromagnetic force, .Iaddend.saidmethod comprising the steps of:detecting a frequency of variations inrotating speed or torque of an output shaft of said.[.electromagnetic.]. clutch .Iadd.means.Iaddend.; detecting an ignitionfrequency of said engine; comparing the detected frequency of variationsin rotating speed or torque of said output shaft of said.[.electromagnetic.]. clutch .Iadd.means.Iaddend., with the detectedignition frequency of said engine; and controlling an engaging action ofsaid .[.electromagnetic.]. clutch .Iadd.means .Iaddend.to adjust aclutch torque transmitted by said clutch .Iadd.means.Iaddend., based onwhether the detected frequency of variations in rotating speed or torqueof said output shaft is coincident with the detected ignition frequencyof said engine.
 5. An apparatus for controlling .[.an electromagnetic.]..Iadd.a .Iaddend.clutch .Iadd.means having an electromagnetic coil.Iaddend.incorporated in a power transmission system of a vehicle totranmit an output of an engine to drive wheels of the vehicle, so as toabsorb a variation in output torque of said engine, an engaging actionof said clutch .Iadd.means .Iaddend.being controlled to adjust a torquetransmitted by said clutch .Iadd.means .Iaddend.such that said clutchtorque is varied as a function of an amount of electric current .Iadd.orvoltage .Iaddend.applied to the .[.electromagnetic.]. clutch.Iadd.means.Iaddend., comprising:first detecting means for detecting anignition frequency of said engine; second detecting means for detectinga frequency of variations in rotating speed of an output shaft of said.[.electromagnetic.]. clutch .Iadd.means.Iaddend.; and current adjustingmeans for controlling said amount of electric current .Iadd.orvoltage.Iaddend., said current adjusting means reducing said amount ofelectric current .Iadd.or voltage .Iaddend.when said frequency ofvariations is coincident with said ignition frequency, and increasingsaid amount of electric current .Iadd.or voltage,.Iaddend.when saidfrequency of variations is not coincident with said ignition frequency.6. An apparatus for controlling .[.an electromagnetic.]. .Iadd.a.Iaddend.clutch .Iadd.means having an electromagnetic coil.Iaddend.incorporated in a power transmission system of a vehicle totransmit an output of an engine to drive wheels of the vehicle, so as toabsorb a variation in output torque of said engine, an engaging actionof said clutch .Iadd.means .Iaddend.being controlled to adjust a torquetransmitted by said clutch means such that said clutch torque is variedas a function of an amount of electric current .Iadd.or voltage.Iaddend.applied to the .[.electromagnetic.]. clutch.Iadd.means.Iaddend., comprising:first detecting means for detecting anignition frequency of said engine; second detecting means for detectinga frequency of variations in torque of an output shaft of said.[.electromagnetic.]. clutch .Iadd.means.Iaddend.; and current adjustingmeans for controlling said amount of electric current .Iadd.orvoltage.Iaddend., said current adjusting means reducing said amount ofelectric current .Iadd.or voltage .Iaddend.when said frequency ofvariations is coincident with said ignition frequency, and increasingsaid amount of electric current .Iadd.or voltage .Iaddend.when saidfrequency of variations is not coincident with said ignition frequency.7. A method of controlling .[.an electromagnetic.]. .Iadd.a.Iaddend.clutch .Iadd.means .Iaddend.incorporated in a powertransmission system of a vehicle to transmit an output of an engine todrive wheels of the vehicle, so as to absorb a variation in outputtorque of said engine, .Iadd.a clutch torque transmitted by said clutchmeans being varied through an electromagnetic force, .Iaddend.saidmethod comprising the steps of:detecting the number of ignitions perunit time of said engine; detecting the number of variations per unittime in torque of an output shaft of said .[.electromagnetic.]. clutch.Iadd.means.Iaddend.; comparing the detected number of ignitions perunit time of said engine with the detected number of variations per unittime in torque of said output shaft; and controlling an engaging actionof said .[.electromagnetic.]. clutch .Iadd.means .Iaddend.to transmit aclutch torque transmitted by said clutch .Iadd.means.Iaddend., such thata difference between said detected number of ignitions of the engine andsaid detected number of variations in torque of said output shaftcoincides with a predetermined reference value.
 8. A method as recitedin claim 7, wherein said number of variations per unit time in torque ofsaid output shaft is counted each time a waveform representative of thetorque of said output shaft exceeds a predetermined threshold level. 9.A method as recited in claim 7, wherein said predetermined referencevalue is determined so as to permit maximum absorption of said variationin output torque of said engine with substantially no power loss withsaid .[.electromagnetic.]. clutch .Iadd.means.Iaddend..
 10. A method asrecited in claim 7, wherein said predetermined reference value isadjusted based on a speed of said engine.
 11. An apparatus forcontrolling .[.an electromagnetic.]. .Iadd.a .Iaddend.clutch .Iadd.meanshaving an electromagnetic coil .Iaddend.incorporated in a powertransmission system of a vehicle to transmit an output of an engine todrive wheels of the vehicle, so as to absorb a variation in outputtorque of said engine, an engaging action of said clutch .Iadd.means.Iaddend.being controlled to adjust a clutch torque transmitted by saidclutch .Iadd.means .Iaddend.such that said clutch torque is varied as afunction of an amount of electric current .Iadd.or voltage.Iaddend.applied to the .[.electromagnetic.]. clutch.Iadd.means.Iaddend., comprising:.[.first detecting means for detectingthe number of ignitions per unit time of said engine; seconddetectinutch, comprising:.]. first detecting means for detecting thenumber of ignitions per unit time of said engine; second detecting meansfor detecting the number of variations per unit time in torque of anoutput shaft of said .[.electromagnetic.]. clutch .Iadd.means.Iaddend.;and current adjusting means for controlling said amount of electriccurrent .Iadd.or voltage .Iaddend.such that a difference between saidnumber of ignitions of the engine and said number of variations intorque of said output shaft coincides with a predetermined referencevalue. .Iadd.
 12. An apparatus as recited in claim 11 wherein saidnumber of variations per unit time in torque of said output shaft iscounted each time a waveform representative of the torque of said outputshaft exceeds a predetermined threshold level. .Iaddend. .Iadd.13. Anapparatus as recited in claim 11 wherein said predetermined referencevalue is determined so as to permit maximum absorption of said variationin output torque of said engine with substantially no power loss withinsaid clutch means. .Iaddend. .Iadd.14. An apparatus as recited in claim11 wherein said predetermined reference value is adjusted based on aspeed of said engine. .Iaddend. .Iadd.15. An apparatus for controlling aclutch means incorporated in a power transmission system of a vehiclefor transmitting output torque of an engine to drive wheels of thevehicle, so as to absorb a variation in output torque of said engine, aclutch torque transmitted by said clutch means being varied through anelectromagnetic force, said apparatus comprising:means for detecting avariation in rotating speed or torque of an output shaft of said clutchmeans; and means for controlling an engaging action of said clutch meansto adjust said clutch torque transmitted by said clutch means, based onwhether the detected variation in rotating speed or torque of saidoutput shaft of said clutch means is synchronized with an ignitionperiod of said engine. .Iaddend. .Iadd.16. A method for controlling aclutch means having an electromagnetic coil incorporated in a powertransmission system of a vehicle for transmitting output torque of anengine to drive wheels of the vehicle, so as to absorb a variation inoutput torque of said engine, an engaging action of said clutch meansbeing controlled to adjust a torque transmitted by said clutch meanssuch that said clutch torque is varied as a function of an amount ofelectric current or voltage applied to the clutch means, said methodcomprising the steps of: detecting an ignition frequency of said engine;detecting a frequency of variations in torque of an output shaft of saidclutch means; and controlling said amount of electric current or voltageby reducing said amount of electric current or voltage when saidfrequency of variations is coincident with said ignition frequency, andincreasing said amount of electric current or voltage when saidfrequency of variations is not coincident with said ignition frequency..Iaddend.